Incremental browser-based device fingerprinting

ABSTRACT

A method for incrementally fingerprinting a computing device includes steps for executing device-identification code programmed to retrieve a complete set of device configuration metrics from a web browser of the computing device, interrupting execution of the device-identification code after retrieval of a first subset of the device configuration metrics, generating a partial device fingerprint from the first subset, generating a temporary device identifier from the partial device fingerprint, and generating a subset indicator identifying the device configuration metrics that compose the first subset. Additional steps include writing the temporary device identifier and the subset indicator to browser-based storage of the computing device, resuming execution of the device-identification code, querying the browser-based storage for presence of the temporary device identifier, and retrieving, responsive to detecting the temporary device identifier, a second subset of the device configuration metrics, the second subset comprising device configuration metrics not identified by the subset indicator.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.14/035,580, filed on Sep. 24, 2013, which is a continuation of U.S.application Ser. No. 13/290,549, filed on Nov. 7, 2011, now U.S. Pat.No. 8,601,109, which claims priority to U.S. Provisional Application No.61/410,815 filed Nov. 5, 2010, the disclosure of which is herebyincorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to methods and apparatus for uniquelyidentifying remote computing devices. More specifically, the inventionis directed to fingerprinting a remote computing device in stages usinginformation retrievable from a web browser.

2. Description of the Related Art

Device identification is used in network communications, for example, toallow a server to uniquely identify remote computing devices thatrequest access to information or services available through the server.There are many applications for device identification, includingsecurity, remote licensing of software, and targeted advertising ofweb-deliverable content, to name a few.

Device identification may involve collecting information from a remotecomputing device to generate a “fingerprint” for the device. A devicefingerprint, as the name implies, consists of computer-readable codethat uniquely identifies a computing device. Any computing devicecapable of communicating with other computing devices, for example, alaptop computer or mobile phone equipped for Internet communications,may be uniquely fingerprinted using methods known to the assignee of thepresent application and disclosed in U.S. Pat. No. 5,490,216, U.S.patent application Ser. Nos. 11/531,235 and 11/531,257 filed Sep. 12,2005, and U.S. patent application Ser. Nos. 12/903,948 and 12/903,980filed Oct. 13, 2010, all of which are fully incorporated herein byreference.

Device-based fingerprinting works by collecting device identificationmetrics (also known as indicators or computer-readable parameters) froma target device, for example, data stored on the device that representsan manufacturer-provided identification number, manufacturer name,serial number, version number, or performance specification, forhardware devices installed on or peripheral to the device, or otheruser-configurable or non-user-configurable parameters. Another type ofdevice fingerprinting, web-based or browser-based fingerprinting, worksby collecting indicators from data accessible from a web browser runningon the target device. In either case, a special program may be executedto collect the data, and may be resident in device-based storage, i.e.,memory stored locally on the device, or the program may be run at aserver remote from the target device that queries the target device forparameters accessible to the server, for example, via a web browserrunning on the target device. When the fingerprint is taken, a uniqueidentification code based on the fingerprint may be stored locally onthe target device, for example, as a flash cookie. In subsequenttransactions committed by the same target device, a server may quicklyand easily identify the device by reading the unique identificationcode, without having to re-fingerprint the device.

The fingerprinting program may often be attached to a web page, and runin the background while a target or client device visits the website tobrowse or to effect some transaction. A problem that often arises when abrowser-based fingerprinting program runs from a web page is that ittakes some amount of time to collect all of the indicators that areneeded to establish a unique identity for the target device. This maycause undesirable loading delays when displaying the web page, which canbe too slow for many market applications, or can otherwise detract froma visitor's experience or even drive the visitor away. When visitorsnavigate away from a web page before the fingerprinting indicators havebeen collected, it may not be possible to establish a unique identifierfor the target device.

Browser-based fingerprinting can be further complicated by targetdevices equipped with multiple browsers. Such a device, initiallyfingerprinted when running a first browser, may appear to thefingerprinting program to be a different device when running a secondbrowser. The process of web-based device fingerprinting, therefore, mustcollect sufficient indicators to uniquely identify a device, must runfast enough to meet a host's timing expectations, and must be able toverify the identity of a device previously fingerprinted, regardless ofwhich browser is running. The timing difficulties and the problemspresented by multiple browsers need to be overcome before devicefingerprinting can be made attractive for wide scale use.

SUMMARY OF THE INVENTION

The present invention addresses the difficulties just described byproviding a method for incrementally fingerprinting a computing device,so that the indicators needed to generate the fingerprint may becollected at different times or during different sessions and laterconsolidated. Device-identification code that collect the indicatorsfrom a web browser may be customized according to the invention so thatno single indicator-collection event causes a loading delay that exceedsthe specifications of a web page host.

In one embodiment, a method according to the invention for incrementaldevice fingerprinting includes an initial step for executing from awebsite, responsive to the website receiving a resource request througha web browser of a computing device, device-identification code foridentifying the computing device, the device-identification codeprogrammed to retrieve a complete set of device configuration metricsfrom the web browser. Subsequent steps of the method includeinterrupting execution of the device-identification code after retrievalof a first subset of the device configuration metrics, generating apartial device fingerprint from the first subset of device configurationmetrics, generating a temporary device identifier from the partialdevice fingerprint, and generating a subset indicator identifying thedevice configuration metrics that compose the first subset. Once thisdata is generated, additional steps are performed for: writing thetemporary device identifier and the subset indicator to browser-basedstorage of the computing device, resuming execution of thedevice-identification code, querying the browser-based storage forpresence of the temporary device identifier, and retrieving, responsiveto detecting the temporary device identifier, a second subset of thedevice configuration metrics, the second subset comprising deviceconfiguration metrics not identified by the subset indicator. A finalstep for updating the partial device fingerprint responsive toretrieving the second subset of the device configuration metrics maycomplete the method.

In another embodiment, upon resuming execution of thedevice-identification code, the method may further include querying thebrowser-based storage for the subset indicator, and if the subsetindicator indicates that all device configuration metrics in the sethave been retrieved, omitting further retrieving steps, and promotingthe temporary device identifier to a permanent device identifier.Promoting the temporary device identifier to permanent may be achievedby marking the subset indicator as permanent in the browser-basedstorage, and by writing data presently stored as the temporary deviceidentifier to device-based storage on the computing device in a locationreserved for the permanent device identifier.

In another implementation of the invention for incremental devicefingerprinting, a method for generating a browser-based fingerprint fora computing device is embodied in executable device-identification code.The device-identification code is deployed for execution from a websitein response to the website receiving a resource request through a webbrowser running on the computing device. When executed, thedevice-identification code performs the following method steps: (a)querying browser-based storage of the requesting web browser for apermanent device identifier indicator, (b) in response to returning nopermanent device identifier indicator, retrieving from the requestingweb browser a subset of a complete set of device configuration metrics,(c) generating a partial device fingerprint from all deviceconfiguration metrics previously retrieved from the requesting webbrowser, (d) generating a temporary device identifier from the partialdevice fingerprint, (e) generating a temporary device identifierindicator identifying all device configuration metrics previouslyretrieved from the requesting web browser, (t) writing the temporarydevice identifier and the temporary device identifier indicator to thebrowser-based storage, (g) re-performing the steps beginning at step (a)to retrieve device configuration metrics not previously retrieved untilthe requesting web browser exits the web page or until the complete setof device configuration metrics has been retrieved. When the completeset of device configuration metrics has been retrieved, additionalmethod steps are executed for: (h) generating a complete devicefingerprint from the complete set of device configuration metrics, (i)generating a permanent device identifier from the full devicefingerprint, (j) writing the permanent device identifier to device-basedstorage on the computing device, and (j) writing a permanent deviceidentifier indicator to the browser-based storage. The method may befurther characterized in that an initial subset may consist of thosedevice configuration metrics that satisfy the following conditions: (i)the metrics are retrievable from the web browser within a time periodspecified by the website, and (ii) the metrics provide the highestprobability among all subsets satisfying condition (i) of uniquelyidentifying the computing device.

BRIEF DESCRIPTION OF THE DRAWINGS

Other systems, methods, features and advantages of the invention will beor will become apparent to one with skill in the art upon examination ofthe following figures and detailed description. It is intended that allsuch additional systems, methods, features and advantages be includedwithin this description, be within the scope of the invention, and beprotected by the accompanying claims. Component parts shown in thedrawings are not necessarily to scale, and may be exaggerated to betterillustrate the important features of the invention. In the drawings,like reference numerals designate like parts throughout the differentviews, wherein:

FIG. 1 is a pictorial block diagram of a modern communications networkin which the present invention may be implemented.

FIG. 2 is a block diagram of one embodiment of server systems used toimplement the present invention.

FIG. 3 is a block diagram of software components within a computingsystem for running a browser-based full fingerprinting program withineither of two different web browsers according to one embodiment of theinvention.

FIG. 4 is a block diagram of software components according to theinvention within a computing system for running a browser-basedincremental fingerprinting program within either of two different webbrowsers according to one embodiment of the invention.

FIG. 5 is a process flow chart illustrating one embodiment of a methodaccording to the invention for incrementally fingerprinting a computingdevice.

FIG. 6 is a process flow chart illustrating parallel processes beingexecuted in a web page running an incremental fingerprinting programaccording to one embodiment of the invention.

FIG. 7 is a process flow chart illustrating a method according to theinvention for incremental browser-based device fingerprinting, includingsteps for promoting a temporary device identifier to a permanent deviceidentifier.

FIG. 8 is a process flow chart presenting salient steps in a methodaccording to the invention for incremental browser-based devicefingerprinting.

FIG. 9 is a process flow chart presenting salient steps in anothermethod according to the invention for incremental browser-based devicefingerprinting.

DETAILED DESCRIPTION

The present invention provides a method for fingerprinting a computingdevice in a stages using information obtainable from a special computerprogram running within a browser of the computing device. The inventiondiscloses an incremental browser-based fingerprinting program thattypically runs while the computing device is downloading a web page. Theprogram avoids causing prolonged loading delays by providing thecapability for distributing the total execution time needed to extract acomplete set of fingerprinting metrics over multiple browsing sessions.

Within the scope of the invention, computing devices that may beuniquely identified with a device fingerprint include any device capableof receiving resources remotely through a network connection. FIG. 1illustrates many such devices connected in a modem networkcommunications system 10. System 10 represents but one example of anetwork within which the present invention may be practiced.

System 10 at its center depicts a network cloud 11, which represents acombination of wired and wireless communication links between devicesthat make up the rest of the system. The communication links of network11 may run from any device to any other device in the network, and mayinclude any means or medium by which analog or digital signals may betransmitted and received, such as radio waves at a selected carrierfrequency modulated by a signal having information content. Network 11may include telecommunication means such as cellular communicationschemes, telephone lines, and broadband cable. The communication meansof network 11 may also include any conventional digital communicationsprotocol, or any conventional analog communications method, fortransmitting information content between computing devices. In oneembodiment, or for ease of illustration, network 11 may be considered tobe synonymous with the Internet.

Incremental device fingerprinting for any device connected to network 11may be performed by running an executable device-identification code,also known as incremental fingerprinting (IFP) code, on the connecteddevice. The IFP code may be stored on any device or number of devices;however, for purposes of illustration, throughout the remainder of thisdisclosure embodiments of the invention are described in which the IFPcode is stored primarily on an application server 13. When authorized orrequested by a user of any other device connected to network 11, the IFPcode may be transferred from application server to the requesting devicefor execution thereon and for temporary or secondary storage therein.For example, the IFP code may be run in a web browser of the devicebeing fingerprinted.

Application server 13 may be a special-purpose computer system thatincludes a set of hardware and software components dedicated to theexecution and distribution of the IFP code. Application server 13 may beconfigured for network communications, i.e., for transmitting andreceiving resource requests to and from other devices linked to network11, and may include a web server to facilitate network communications.Application server 13 may also be configured to perform other functionsconventionally associated with application servers, such as security,redundancy, fail-over, and load-balancing. A user interface 15 providesuser or administrator access to data processed by the applicationserver, or to the software components that make up the applicationserver. Memory 17 stores operating system, web server, IFP code, andother data or executable software stored on application server 13.

A database server 19 may be linked for data communication withapplication server 13. Database server 19 may be a special purposecomputer system that includes hardware and software components dedicatedto providing database services to application server 13. Database server19 interfaces with memory 21, which may be a large-capacity storagesystem. In one implementation of incremental device fingerprintingaccording to the invention, memory 21 may be a main repository orhistorical archive for storing a comprehensive collection of devicefingerprints for millions of devices communicating, or having oncecommunicated, through network 11.

Any computing device capable of receiving digital information vianetwork 11 may be subject to incremental device fingerprinting accordingto the invention. System 10 provides a representative group of suchdevices for purposes of illustrating exemplary embodiments of theinvention, but the invention is by no means limited to the number andtype of devices shown in FIG. 1. Examples of devices known today forwhich a device fingerprint may be defined by the invention include, butare not limited to, a PDA 23, a PC 25, a laptop 27, an iPad 29, a smartphone 31, a cell phone 33, and an Apple computer 35, as shown, all orany of which may be configured for direct or indirect communication vianetwork 11. Any device in the preceding list of devices may be referredto hereinafter as a “computing device”, a “client device”, a “requestingdevice”, or a “receiving device”.

A server 37 may also constitute a computing device subject toincremental device fingerprinting. Moreover, each device among a groupof devices configured to communicate locally with server 37, and toaccess network 11 via server 37, may potentially be fingerprintedaccording to the invention. These include, for example, the Applecomputer 35, a PC 39, and a cell phone 43, as shown. Server 37 may beany type of server, such as an application server, a web server, or adatabase server, and may access a memory 41. In one embodiment, server37 provides a web page accessible through network 11 by other devices.The web page may provide information such as text, graphics, datastructures, audio, video and computer applications that are stored asdigital data in memory 41 for downloading or streaming via network 11.

FIG. 2 illustrates a block diagram of a system according to oneembodiment of the invention for implementing incremental devicefingerprinting. As indicated in the figure, the platform of applicationserver 13 may include a set of software programs stored in the memory17. These programs include an operating system (OS) 45, which managesbasic hardware functions of server 13, such as input and output andmemory allocation. OS 45 may further provide services for execution ofapplication software, such as the web server 47 and the IFP 49. OS 45may be any operating system, such as a Windows, Unix, Mac OS, Linux, orGoogle Chrome operating system.

Web server 47 may comprise software necessary for serving content usingan appropriate protocol, such as hypertext transfer protocol (HTTP),over network 11. The content served may include a web page to facilitatecommunication with other devices via the network. In any event, webserver 47 is configured to respond to resource requests received fromclient devices, and to issue resource requests to client devices. Forexample, web server 47 may include routines for requesting resourcesfrom other devices or servers connected to network 11. A resourcerequest may be fulfilled by web server 47, for example, by uploading IFP49 to a requesting server, or to a client on behalf of a requestingserver, for incremental device fingerprinting on the client deviceaccording to the invention. Web server programs having user agentapplications implementing network protocols for connecting to uniformresource locator (URL) paths are well known in the computing arts, andany such program, supplemented as necessary for implementation of IFP 49of the present invention, may be appropriate for the coding of webserver 47.

Software program IFP 49 includes all code, data, text files, andexecutable algorithms for deriving a partial device fingerprint or afull device fingerprint from a computing device. IFP 49 may be organizedas a source tree, and may be composed in any source code languagecompatible with modern computing systems, such as Java or C, and mayimplement encoding rules such as XML. IFP 49 may further include data,executable files, plug-ins, or other content that may comprise resourcesor requests served by web server 47 via network 11 under control of OS45. An incremental device fingerprinting algorithm stored in IFP 49 mayderive a device fingerprint for any remote computing devicecommunicating via network 11, such as any one of devices 23, 25, 27, 29,31, 33, 35, 37, 39, and 43. Examples of IFP algorithms dedicated toderiving device fingerprints from data associated with a particularremote computing device are modeled and described herein as process flowdiagrams 400, 500, 600, 700, 800 and 900.

The platform of database server 19 may be stored in memory 21. Memory 21may store an operating system (OS) program 51, as well as database (DB)server software 53. The OS 51 may be any operating system satisfactoryfor this purpose, such as any of those listed above for applicationserver 13. DB server software 53 contains the programs necessary forproviding database services to application server 13. The memory 21 mayalso provide large-scale data storage for a master archive of devicefingerprints. Memory 21 may also provide backup storage, such as in amaster-slave architecture for database servers. The application server13 and database server 19, interacting with remote devices via network11 within a system such as system 10, provides an exemplary computingenvironment for incremental device fingerprinting according to theinvention.

To better understand the utility of incremental fingerprinting, anexplanation of full fingerprinting is first provided. FIG. 3 is a blockdiagram 300 of software components within a computing device 25implementing a browser-based full fingerprinting program. Fullfingerprinting is the eventual goal when fingerprinting a computingdevice. A full fingerprint is obtained when the fingerprinting programhas collected all metrics from a target device that it is designed tocollect, and from those metrics generates a complete fingerprint for thedevice that uniquely identifies the device among millions of othercomputing devices, e.g. the total population of devices connectable tonetwork 11. In this sense, a fingerprinting program according to theinvention is one that uniquely identifies a device so that theprobability of the program generating an identical fingerprint foranother device is less than a desired acceptance value. In oneembodiment, the desired acceptance value is 1×10⁹ (one in one billion).Other acceptance values are possible within the scope of the invention.

Computing device 25 includes many features not shown in the figure thatwould be readily understood by an experienced computer software engineeras inherent properties of the computer. These include conventionalhardware components, e.g. core processor, memory, power supply,interface cards, and communication ports, and software components, e.g.BIOS, operating system, graphical user interface, and other applicationsthat manage hardware functions, perform memory allocations, read andwrite operations, etc. The operating system of PC 25 may be any of thesame proposed for OS 45. The operating system may further provideservices for execution of application software, such as web browsers andthe fingerprinting programs disclosed herein.

A browser-based fingerprinting program according to the invention may berun effectively on a computing device 25 equipped with any number ofbrowsers. For ease of illustration, the invention is described hereafterwithin the context of a computer running two different browsers, eachpreferably running at different times. The principles of the inventionmay be scaled for computing devices running more than two browsers, andmay be applied to devices running multiple browsers simultaneously. Indiagram 300, PC 25 includes two different web browsers. Web browser 55is arbitrarily denoted as type A, and web browser 57 is denoted as typeB to distinguish it from type A. For example, browsers 55 and 57 may beversions of Microsoft Internet Explorer and Mozilla Firefox,respectively.

With reference to diagram 300, a user of device 25 with an activeconnection to network 11 may, through one of the browsers 55 or 57,request one resources from one or more remote servers and cause websites59 to return resources to the device. When the user request” atriggering resource using browser 55 or 57, a full fingerprintingprogram 60 will load. Program 60 runs in the background and begins tocollect metrics that are detectable within the open browser. When arequisite number of metrics are collected, a full fingerprint 63 or 65(depending on which browser is open) is generated by some combination ofsampling, hashing, encoding, and combining binary data that composes orrepresents each of the metrics. The full fingerprint may then betransmitted back to the server from which device 25 requested thetriggering resource, or transmitted for storage on a third party serverfunctioning as a fingerprint archive (e.g. server 19).

On device 25, program 60 then creates a permanent device identifier(IDP) 68, which is a data string that references the full fingerprint.Program 60 may then store the permanent device identifier 68 indevice-based storage (DBS) 67, i.e. memory local to the device. Oneexample of device-based storage would be memory on a hard drive reservedfor use by the web browser. To store the fingerprint in device-basedstorage 67, program 60 must first gain access to the storage location.In one implementation, access to device-based storage 67 may be effectedby program 60 loading on the requesting device a program such as Flashor Silverlight that runs outside the browser. In one embodiment, one IDP68 may be stored as a Flash cookie, i.e. storage at the device levelaccessible to a browser capable of running Flash.

There are, however, a number of uncertainties that may arise using thebrowser-based full fingerprinting method. For one, the time required forFP program 60 to collect metrics for a full fingerprint can easilyexceed the time available for such collection. The brevity of thecollection window may result from the user navigating away from the website or closing the browser, or it may result from a strictspecification imposed by the web page host. For example, in markets suchas ad delivery, where rendering speed carries a high priority, a hostmay restrict total loading or execution time of a fingerprinting programto 40 milliseconds or less. Having to load flash or otherwise gainaccess to device-based storage can slow the process even further. As aresult, there may be insufficient time for generating the fullfingerprint 63 or 65.

Another uncertainty can occur when a user accesses the server in asubsequent browsing session using a different browser than the browserthat was used to collect a full fingerprint 63 in an earlier session. Inthis scenario, a full fingerprint 65 generated by FP program 60 runningin the second browser 57 may appear to identify an entirely differentcomputing device than the device identified by full fingerprint 63 inbrowser 55, even though fingerprints 63 and 65 both identify the samecomputing device 25. The extent of the differences between the twofingerprints will depend on how differently the browsers are configured,in terms of the metrics collected by FP program 60. Certain metrics thatare collectable via a browser, such as an IP address, may not change asa result of loading one browser or another. Others metrics may vary,depending on the browser type and configuration.

In various embodiments, the list of device configuration metricscollectable or retrievable by an IFP running a device identificationcode according to the invention may be selected from the followingtable:

TABLE 1 LIST OF CONFIGURATION METRICS TYPE METRIC PDF (a) contains aproperty “SupportsAcrobat” which will contain the description stringfrom the navigator plug-ins object or objects which contain the words“adobe acrobat” or “adobe reader”. AJAX (a) contains string“XMLHttpRequest object” if the browser is able to instantiateXMLHTTPRequest object. (b) contains string “ActiveX object (Msxm12)” ifinstantiation of XMLHTTPRequest object fails and Msxml2.XMLHTTP can beinstantiated. (c) contains string “ActiveX object (Microsoft)” ifinstantiation of previous 2 components fails. (d) contains empty stringif all (a), (b) and (c) fail. CPU (a) “unknown” if cannot be determined.(b) contains navigator.cpuClass if available. (c) contains“wow64-”navigator.cpuClass if WOW is detected. TIMEZONE (a) containsproperty “Offset” which indicates timezone offset from GMT in minutes.FONTS (a) contains list of fonts returned from flash control if Flash isavailable. (b) if Flash is not available, contains a list of fonts asdetected by javascript (these fonts are detected by checking themachine's fonts against a list in the javascript and in general willdetect a subset of the fonts actually installed). GEARS (a) containsempty string if gears is not installed, otherwise contains result ofgoogle.gears.factory.getBuildlnfo( ); an example of which is the string“0.5.36.0; official; opt; win32; ie”. LANGUAGES (a) contains propertyLanguage which is result of navigator.language, property “SystemLanguage” which is result of navigator.systemLanguage, property “UserLanguage” which is result of navigator.userLanguage and “BrowserLanguage” property which is result of navigator.browserLanguage. PLUGINS(a) contains a set of CLSID/version properties; CLSIDS comes from afixed list of CLSIDs on IE and from the list of plugins returned bynavigator.plugins on other platforms (on platforms other than IE, therewill be name/description/filename property sets for each plugin) GENERAL(a) contains properties based on each of the following values:navigator.appName + navigator.appCodeName (note these first 2 are notseparated by a space) navigator.appVersion + navigator.appMinorVersion;navigator.vendor; navigator.userAgent; navigator.oscpu;navigator.platform; navigator.securityPolicy; navigator.onLine;SILVERLIGHT collected through attempts to instantiate the SilverlightActiveX COMPONENTS control and loop through a fixed list of versions andvia the IsSupportedVersion( ) method on the Silverlight controldetermine which versions are available: (a) if versions are available,property “Version” will be added for each Silverlight versiondiscovered. (b) If an error occurs, a version property will be addedwith the value “Error”. READER contains the PDF reader componentsavailable on the machine. COMPONENTS DOTNET retrieves Microsoft .Netversions detected by examining the user agent string. GUID contains aservice provider-assigned GUID contained device storage such as a Flashcookie. DISPLAY contains two properties - width and height whichindicate the display resolution in pixels. IP ADDRESS (a) contains theIP address of the device which requested the page (this could be aproxy, it could be a NAT, or it could be the user's machine). (b) on theserver side, the IP address is geolocated. If available, the country,zip code and/or telephone area code associated with the IP address areused in place of the actual IP address.

It should be understood that the foregoing list may expand or changealong with advances in the art that make available for collectionthrough a browser additional configuration metrics that are not in usetoday or may be otherwise unknown to the inventor at this time.

FIG. 4 illustrates a block diagram 400 of a browser-based incrementalfingerprinting scheme in accordance with the present invention.Incremental fingerprinting may be executed by an incrementalfingerprinting (IFP) program 70 running within a browser a computingdevice 25. As in the previous example, only two browsers (55 and 57),each of a different browser type, are illustrated in diagram 400. Theinvention may be scaled to accommodate any number of different browsertypes residing on a common computing device.

Incremental browser-based fingerprinting is designed to avoid excessiverendering times associated with loading Flash or any other technologythat resides outside the browser. Rather than relying entirely ondevice-based storage, the invention exploits the capabilities ofbrowser-based storage (BBS). Browser-based storage refers to memory thatis specifically provided by, or integrated within the browser processingmodule, as opposed to memory external to the browser that needs to beseparately loaded each time it is accessed. One example of browser-basedstorage is DOM-based storage. DOM-based storage is known in the art, andhas been employed in different browser software. For example, MicrosoftInternet Explorer employed DOM-based storage in version 5.5. Otherexamples of browser-based storage, such as HTML-5, may be employed inaccordance with the present invention.

To enable a more responsive execution time, an incrementalfingerprinting process according to the invention collectsfingerprinting metrics in stages or increments. Generally, the order inwhich the metrics are collected is determined by a comparative scalehaving two criteria: (i) indication of uniqueness and (ii) speed ofcollection. In the earliest stages, metrics targeted for collection arethose metrics that are most indicative of the uniqueness of a computingdevice and that can be collected the most quickly. In later stages,additional metrics are targeted that are either less indicative ofuniqueness or less able to be collected quickly. In the final stages,the targeted metrics are those that are less indicative of uniquenessand less able to be collected quickly. By collecting metrics this order,the highest quality partial fingerprint may be collected in the initialstage. In subsequent stages, additional metrics are collected tosupplement the fingerprint, until eventually all targeted metrics arecollected for generation of a full fingerprint. By collecting metrics instages, the execution time of each stage may be controlled to conform tohost specifications and avoid excessive loading delays.

Implementation of incremental browser-based fingerprinting is madepossible through IFP 70 accessing browser-based storage 75. IFP 70comprises the executable device identification code that is programmedto retrieve a complete set of device configuration metrics from withinthe web browser in which it runs. When IFP 70 runs in its initial stage,it quickly collects a first subset of the complete set of metrics. Atsome point, the execution of the device-identification code of IFP 70 isinterrupted, for example, as result of a user closing browser 55, as aresult of expiration of an allotted execution time, or as a result ofsuccessful collection of all metrics that compose the first subset. IFP70 will then generate incremental fingerprint 71, which is a partialdevice fingerprint based on the device configuration metrics that werecollected as the first subset. The metrics that compose the first subset(or other subsets) may be determined according to various criteria. Inone example, a subset may consist of the number of device configurationmetrics that have been retrieved when the web browser exits the webpage. In another example, a subset may consist of the minimum number ofdevice configuration metrics needed to satisfy a predeterminedfingerprinting criteria such as identification accuracy. In anotherexample, a subset may consist of the number of configuration metricsretrieved upon expiration of an allotted execution time.

In one embodiment, incremental fingerprint 71 comprises discrete datastrings, each data string representing a collected metric. In anotherembodiment, an incremental fingerprint may comprise a concatenation orcombination of two or more such data strings. In another embodiment, theincremental fingerprint may comprise a data file, such as a text or XMLfile encoded in computer-readable form and including each collectedmetric. IFP 70 may include a hashing algorithm that transforms the datathat makes up the incremental fingerprint. In another implementation,IFP 70 may cause incremental fingerprint 71 to be transmitted, forexample, as an encrypted token, to a remote server for storage in afingerprint database. This ensures that the fingerprint can be recordedin a persistent archive.

After the incremental fingerprint is generated, IFP 70 then generates atemporary device identifier (MT) 78. In one embodiment, temporary deviceidentifier 78 comprises a data string that references the incrementalfingerprint. IFP 70 also generates a subset indicator (SI) 79, which maybe another data string that identifies the device configuration metricsthat compose the first subset. For example, each metric in the completeset may be numerically coded in order of importance or according tometric type. Subset indicator 79 may comprise a concatenated string ofthe numerical codes that collectively represent the metrics in thesubset. When these data are generated, IFP 70 may then store thetemporary device identifier 78 and the subset indicator 79 inbrowser-based storage 75. IFP 70 may also transmit the subset indicator79 to the remote server for storage in the fingerprint database andassociation with incremental fingerprint 71. In one embodiment, writingthe temporary device identifier and the subset indicator tobrowser-based storage completes the first partial fingerprinting stage.

At a later time, execution of the device identification code of IFP 70will resume. It may resume immediately after completion of the firstpartial fingerprinting stage, or it may resume after a programmed delay.In another scenario, the execution resumes when the browser revisits thetriggering web page, which is described in further detail below. In anycase, when IFP 70 resumes, it queries the browser-based storage todetect the presence of a temporary device identifier 78. If in responseto the query, the temporary device identifier is detected or returned,IFP 70 then retrieves the partial device fingerprint associated with thetemporary device identifier and commences collecting a second set ofdevice configuration metrics. In one embodiment, the second set ofmetrics includes metrics of the next-highest level of importance on thecomparative scale, and includes only metrics that are not included inthe first subset. By reading subset indicator 79, IFP 70 can detectwhich metrics have already been collected and exclude any of thecollected metrics from the second subset. When the next interruptionoccurs in the execution of the device identification code, eitherthrough termination of browser 55 or through IFP 70 satisfying a processcriterion, IFP 70 may update the partial device fingerprint 71 using thenewly collected metrics. Updated values for the temporary deviceidentifier 78 and subset indicator 79 are then generated and recorded inbrowser-based storage 75.

The process of incremental fingerprinting may continue in this manner,by collecting subsets of metrics in chronological stages and updated theincremental fingerprint, until the subset indicator 79 is queried andreturns indication that a complete set of device configuration metricshas been collected. At this point, IFP 70 may promote the temporarydevice identifier 78 to a permanent device identifier (IDP) 68. In oneembodiment, promotion of the device identifier includes marking ormodifying the subset indicator 79 to indicate that a full fingerprinthas been taken, and writing the temporary device identifier 78 todevice-based storage 67. When written to device-based storage, thetemporary device identifier 78 represents a permanent device identifier.In one embodiment, the permanent device identifier 68 may be stored as asingle Flash cookie, i.e. data recorded at the device level accessibleto a browser capable of running Flash.

With the subset indicator 79 marked as permanent in browser-basedstorage 75, the computing device need not be fingerprinted a second timein response to a resource request in a subsequent browsing session.Instead, during a subsequent session, IFP 70 may first querybrowser-based storage for the indicator 79. If the indicator is found.IFP 70 may transmit a device identifier 68 or 78 back to the server toidentify the device to the server as a recognized device. The server mayconfirm the identity by matching the device identifier to a fingerprintstored in an accessible archive.

In diagram 400, the second browser 57 is shown also residing on device25. The process of incremental browser-based fingerprinting asimplemented within browser 55 may also be implemented within browser 57.When a user loads a second browser 57 and visits a website configuredwith a scheme for triggering IFP 70, the process of incrementalfingerprinting will commence just as if browser 57 were running on anunknown computing device. IFP 70 will generate and update an incrementalfingerprint 73 in stages, and at the conclusion of each stage write atemporary device identifier 81 and subset indicator 82 to browser-basedstorage 77 within browser 57. Promotion of the partial fingerprint to apermanent fingerprint occurs when subset indicator 82 indicates that acomplete set of configuration metrics has been collected, and temporarydevice identifier 81 is written to DBS 67 as the permanent deviceindicator 68. Incremental fingerprints 73 and subset indicators 82 maybe stored or updated in a remote database between stages.

Incremental fingerprinting may commence within browser 57 whether or notthere has been any prior fingerprinting activity performed for device25. In one scenario, a partial fingerprint has been previously generatedfor device 25 within browser 55 when the user opens the second browser57. In another scenario, a full fingerprint has already been generatedfor device 25 through the first browser 55, and a permanent deviceidentifier stored in DBS 67, when the user opens the second browser 57.In either scenario, incremental fingerprinting may commence withinbrowser 57. To avoid generating two full fingerprints for the samedevice, however, a method according to the invention initiatesasynchronous fingerprint completion, which is described below in furtherdetail.

FIG. 5 presents a flow chart 500 illustrating process steps in a methodaccording to the invention for incremental browser-based devicefingerprinting. Flow chart 500 may be combined with charts 600 and 700to capture the overall process. The process begins at block 502, whichrepresents a user of a computing device such as device 25 visiting a website. By visiting the web site, the user is requesting one or moreresources from the site. The site visited is equipped with anincremental fingerprinting code IFP. In the next block 504, the sitelaunches the IFP code, which enables a device identification algorithmto run within the browser of the user's computing device. The processand decision blocks that follow represent additional features of the IFPcode. The next step 506 is a decision block that determines whether adevice identifier is already present in browser-based storage. This maybe ascertained in any number of ways, for example, by searchingbrowser-based storage for data residing in a memory location reservedfor the device identifier, or for searching for a data string havingspecialized indicia that identifies it as a device identifier.

If block 506 finds a device identifier, the process moves to block 508.Block 508 is another decision block. It determines whether the deviceidentifier has been marked as permanent. If the device identifier ispermanent, the process moves to block 510, otherwise, the process movesto block 512. A device identifier may be determined to be temporary orpermanent, for example, by reading the status indicator 79 or 82. Atblock 510, a permanent device identifier has been detected inbrowser-based storage. Here, the IFP identifies the computing device tothe website (or to a remote server) as a recognized device based on thepermanent device identifier. The website may then use this informationfor some other purpose, such as granting (or denying) access to securedata, approving (or disapproving) an on-line transaction, fulfilling (ordenying) a resource request, etc. The permanence of device identifiermeans that the user's computing device had been previously fullyfingerprinted, and the device identifier recorded in a device-basedstorage technology such as a Flash cookie for future sharing betweendifferent browsers. That is, the permanent device identifier may havebeen stored using any of the browsers resident on the device, and notnecessarily by the browser presently open. In another scenario, thepermanent device identifier may have been generated as a result of anIFP program being launched by a web site other than the site visited bythe user in block 502. From block 510 the process moves to block 602 toinitiate asynchronous fingerprint completion.

If block 508 determines that the device identifier is temporary, i.e.not marked as permanent, then the process advances to step 512. In thisstep, the computing device is identified to the website based on thetemporary device identifier. The website may use this information foranother purpose, such as identifying the device as accurately aspossible by cross-referencing the temporary device identifier to itsassociated partial device fingerprint. The accuracy of suchidentification will depend on how many metrics or subsets of metricswere collected to generate the associated partial fingerprint. In somecases, the partial fingerprint will be sufficient to uniquely identify adevice. In other cases, the partial fingerprint may narrow the field ofpossible matches to only a few devices. From block 512 the process movesto block 602 to initiate asynchronous fingerprint completion.

Referring back to decision block 506, if no device identifier is found,the process advances to block 514. The absence of a device identifiergenerally indicates that the browser presently open is visiting the website for the very first time. The functions represented in blocks 514,516, 518, and 520 reflect steps in the incremental fingerprintingprocess described with reference to diagram 400. In block 514, thedevice identification code of the incremental fingerprinting programcommences partially fingerprinting the device based on a first subset ofdevice configuration metrics. In block 516, a temporary deviceidentifier is generated based on the subset or subsets of all metricspreviously collected. In block 518, the temporary device identifier isupdated or written to browser-based storage and flagged or marked astemporary. In block 520, a subset indicator may be updated or written tobrowser-based storage to identify all metrics used to generate the mostcurrent partial fingerprint. From here, the process loops back to step508, and eventually advances to block 602.

FIG. 6 presents a flow chart 600 that illustrates additional processsteps for incremental browser-based device fingerprinting. The steps inchart 600 provide greater detail for the asynchronous fingerprintcompletion process. Asynchronous fingerprint completion refers to thefact that incremental fingerprinting may be performed in a series ofnon-consecutive or non-contiguous stages, and may involve concurrent oroverlapping attempts to fully fingerprint a device using more than onebrowser. An incremental fingerprinting attempt made within one browsermay be interrupted at any stage, leaving the process dormant untilre-initiated. While dormant, a user of the same computing device mayopen a different browser and initiate another incremental fingerprintingattempt, which may also be interrupted before fully fingerprinting thedevice and writing a device identifier into device-based storage. Chart600 captures multiple parallel processing paths that may occur duringthe overall fingerprinting process.

The asynchronous fingerprinting process begins at step 602 and maycommence while browser activity proceeds along any of three pathsconcurrently. Along the top-most path, in step 604 the browser beginsrendering a requested web page and continues rendering, i.e. downloadingand displaying text, graphics, data, audio, or video files, loadingFlash or other plug-ins, etc., until the browser completes therendering. The next step 606 represents the user interacting with webpage, for example, visually reading the rendered page, entering data,making selections, etc. This interaction continues until the user leavesthe web page at block 618. When the user leaves the web page, anincremental fingerprinting process may be interrupted.

From block 602, a concurrent process for incremental fingerprinting ispresented in the central paths. Blocks 608, 610, and 612 representsubsequent stages for incremental fingerprinting after an initial stagehas been completed (as in blocks 514, 516, 518, 520). The incrementalfingerprinting activity runs in the background while browser renderingand user interaction processes occur. At block 608, a next subset ofdevice configuration metrics are collected for the device beingfingerprinted. The next subset may be predefined according to apreferred order of importance. The temporary device identifier isupdated based on the additional metrics collected. The subset indicatoris updated to identify all metrics collected. Next, in step 610, thenewly collected metrics are sent to the web site or to a remote serveralong with the updated temporary device identifier, and optionally,along with the updated subset indicator. In step 612, the updated subsetindicator and updated temporary device identifier are saved tobrowser-based storage.

The central path process then advances to decision block 614, at whichpoint the process determines whether the user has exited the web page.If so, the process ends at block 618. If not, decision block 616 isexecuted to determine whether all metrics that compose a complete set ofmetrics needed to fully fingerprint the device have been collected. Thismay be ascertained by reading the updated subset indicator recorded inbrowser-based storage. If there are any metrics not yet collected, theprocess loops back to block 608 for collection of the next subset.

If at block 616 it is determined that all device configuration metricshave been collected, the process advances to block 718. In block 718,the temporary device identifier is promoted to a permanent deviceidentifier. The process of promoting a temporary device identifier topermanent may also run in the background during browser rendering anduser interaction. Generally, this is accomplished by accessingdevice-based storage, e.g. loading Flash, then writing the most recentlyupdated temporary device identifier to device based storage, e.g. as aFlash cookie. In one implementation, where a user stays on thetriggering web page long enough for all metrics to be collected withoutinterruption, and long enough for the IFP to access device-basedstorage, a generated device identifier may be promoted to permanent in asingle pass. A more detailed explanation of a promotion process for adevice identifier is presented in chart 700.

FIG. 7 presents the flow chart 700 that illustrates additional processsteps involved in promoting a temporary device identifier to a permanentdevice identifier. At the first process block 702 a user visits a website equipped with an incremental fingerprinting program that launchesan IFP to the browser of the user's computing device. The IFP executes adevice identification code. In process block 704, the code checks thebrowser-based storage of the device for a device identifier. Next, instep 706, the process determines whether the device identifier ispresent. If not, the process advances to block 514 to begin theincremental fingerprinting process. When incremental fingerprinting iscomplete, the process ends at block 722.

If a device identifier in determined absent in block 706, decision 708is invoked, which determines whether the device identifier is marked aspermanent. If so, the process ends at block 722. If the deviceidentifier is not marked as permanent, the process advances to block710. Here, the IFP accesses device-based storage to check for thepresence of a permanent device identifier. Next, decision block 712determines whether a device identifier is present in device-basedstorage, and if so, the process moves to block 714. In block 714, theIFP updates the temporary device identifier in browser-based storagewith the identifier from device-based storage, and marks the updateddevice identifier as permanent.

The path just described, from block 706 to block 714, addresses thescenario where a full fingerprint has been previously generated for adevice using one browser, where a partial fingerprint has been generatedusing a second browser, and where a user at a later time opens thesecond browser and visits a fingerprint-enabled web page that isequipped to cause an IFP to run within the requesting browser. This timethe user visits the web page long enough for the processes in blocks 710and 712 to access device-based storage and locate the permanent deviceidentifier. At that point, if the incremental fingerprinting process isstill running, it is interrupted at block 714 when the temporary deviceidentifier is promoted. After updating the device identifier, theprocess advances to block 716. Block 716 resolves the apparent problemthat the device being fingerprinted may be recognizable at the remotefingerprint archive by pointing to two different recorded fingerprints—acomplete fingerprint that was generated using the first browser, and apartial fingerprint that was generated using the second browser. Inblock 716, the newly promoted permanent device identifier is transmittedto the server (optionally, as an encrypted token) with instructions forlinking or associating the complete and partial fingerprints in such away that they are recognized as identifying the same device. In oneembodiment, the partial fingerprint is erased and all data associatedwith the partial fingerprint in rewritten for association with thecomplete fingerprint that is already recorded for the device. Theprocess then ends at block 722.

Returning now to the decision block 712, if there is no permanent deviceidentifier present in device-based storage, the process advances toblock 718. Here, the IFP copies the temporary device identifier frombrowser-based storage into device-based storage. For example, thetemporary device identifier may be copied from DOM storage to a Flashcookie. Next, in step 720, the device identifier in browser-basedstorage is marked as permanent, thereby completing the promotion. Theprocess then ends at block 722.

FIG. 8 presents a flow chart that captures the salient steps in oneembodiment of a method 800 according to the invention for incrementalbrowser-based device fingerprinting. The steps of method 800 should beinterpreted in view of the various features of the invention presentedin thus far.

Method 800 begins with an initial step 802, which executes a deviceidentification code portion of an incremental fingerprinting program(IFP) running within a browser of a targeted computing device. Next, instep 804, the execution is interrupted after the IFP receives orcollects a first subset of a complete set of device configurationmetrics. The complete set of metrics is the minimum set needed tofingerprint a computing device so that it may be uniquely identified towithin a specified accuracy. In the next step 806, the IFP generates apartial device fingerprint from the first subset of metrics. In the nextstep 808, the IFP generates a temporary device identifier based on thepartial device fingerprint. Next, in step 810, the IFP generates asubset indicator that identifies all of the metrics that compose thefirst subset. Next, in step 812, the IFP writes the temporary deviceidentifier and the subset indicator to browser-based storage within thetargeted device. At a later time, in step 814, the method resumesexecution of the device identification code of the IFP. Next, in step816, the browser-based storage is queried for the presence of atemporary device identifier. In the final step 818, in response todetecting the temporary device identifier, the IFP retrieves or collectsa second subset of device configuration metrics. The second subset ofmetrics is characterized in that no metric within the second subset isidentified by the subset indicator as belonging to the first subset.

FIG. 9 presents a flow chart that captures the salient steps in anembodiment of a method 900 according to the invention for incrementalbrowser-based device fingerprinting. The steps of method 900 should beinterpreted in view of the various features of the invention presentedin thus far.

Method 900 begins at step 902. This step begins the fingerprintingprocess by querying browser-based storage of a browser requesting aresource, for the presence of a permanent device identifier. Thequerying is understood to originate from a web page responding to theresource request. In the next step 904, if no permanent deviceidentifier is returned as a result of the query, then a subset of acomplete set of device configuration metrics needed to completelyfingerprint a device to within a desired level of accuracy is retrievedfrom the browser. Next, in step 906, a partial device fingerprint isgenerated from all device configuration metrics retrieved in theprevious step. Next, in step 908, a temporary device identifier isderived from the partial device fingerprint. In the next step 910, asubset indicator is generated to identify all device configurationmetrics used to generate the partial device fingerprint. Next, in step912, the process writes the temporary device identifier and the subsetindicator to browser-based storage.

The next step 914 calls for all of the previous steps, i.e. 902-912, tobe re-performed to retrieve additional device configuration metrics notpreviously retrieved, until the retrieval process is interrupted, eitherby the browser exiting the web page or by the process successfullyretrieving the complete set of device configuration metrics. Next, instep 916, when all metrics are retrieved, the process generates acomplete device fingerprint from the complete set of metrics. In thenext step 918, the process generates a permanent device identifier fromthe complete device fingerprint, and in the following step 920, thepermanent device identifier is written to device-based storage. In thefinal step 922, the temporary device identifier is promoted by writingthe permanent device identifier to browser-based storage.

For browser-based fingerprinting, the invention addresses the problemsassociated with loading times and with multiple browsers residing on acommon device. The invention also advantageously preserves fingerprintinformation in a secure location remote from computing devicesfingerprinted. Local storage locations such as browser-based anddevice-based storage are subject to erasure and manipulation by users.Even vendor-provided identifiers such as an IP address can be spoofed byknowledgeable hackers. Fingerprinting a device according to methods ofthe present invention, however, provides a far more reliable and securemethod of identifying devices for purposes of remote authentication. Ina scenario where a device identifier is lost or modified on a computingdevice that was previously fingerprinted, the process presented hereinmay be applied to re-fingerprint the device, or to uncover an imposter.For example, if a permanent device identifier is modified by a user, amismatch will occur at the fingerprint archive, the device will betreated as an unknown device, and incremental fingerprinting willcommence with collection of the first subset of configuration metrics.When a permanent device identifier is eventually generated, it willmatch the permanent fingerprint previously recorded in the archive.

The methods described herein may be implemented on a variety ofcommunication hardware, processors and systems known by those ofordinary skill in the computing arts. The various diagrams and flowcharts described in connection with the embodiments disclosed herein maybe implemented or performed in full or in part with a general purposeprocessor, digital signal processor, application specific integratedcircuit, field programmable gate array, or other programmable logicdevice, discrete gate or transistor logic, discrete hardware components,or any combination thereof designed to perform the functions describedherein. A general purpose processor may be a microprocessor, but in thealternative, the processor may be any conventional processor,controller, microcontroller or state machine. A processor may also beimplemented as a combination of any of the aforementioned computingdevices.

The steps of a method, process, program, or algorithm described inconnection with the embodiments disclosed herein may be embodieddirectly in hardware, in a software module executable by a processor, orin a combination of the two, e.g. as firmware. A software module mayreside in memory such as RAM, ROM, EPROM, EEPROM, flash memory,registers, a hard disk, a removable disk, a CD-ROM, or another softwaremodule such as a web browser, or within any other form of storage mediumknown in the art for recording digital data. An exemplary storage mediummay be coupled to the processor, such that the processor can readinformation from, and write information to, the storage medium. In thealternative, the storage medium may be integral to the processor. In apure form, a method according to the invention may be software embodiedas an electronic signal or series of electronic signals capable of beingtransmitted as information wirelessly or otherwise, for example, as amodulating signal receivable through a modem as a downloadable file orbit stream.

Exemplary embodiments of the invention have been disclosed in anillustrative style. Accordingly, the terminology employed throughoutshould be read in an exemplary rather than a limiting manner. Althoughminor modifications to the teachings herein will occur to those wellversed in the art, it shall be understood that what is intended to becircumscribed within the scope of the patent warranted hereon are allsuch embodiments that reasonably fall within the scope of theadvancement to the art hereby contributed, and that that scope shall notbe restricted, except in light of the appended claims and theirequivalents.

What is claimed is:
 1. A method for generating a fingerprint for acomputing device with a web browser requesting a resource from awebsite, the method embodied in device-identification code executing onthe computing device comprising: (a) sending a subset of a complete setof device configuration metrics for the computing device to the website;(b) receiving a temporary device identifier and subset indicator fromthe website; (c) writing the temporary device identifier and subsetindicator to the storage of the computing device; (d) re-performing thesteps beginning at step (a) to send device configuration metrics notpreviously sent to the website until the web browser exits the websiteor until the complete set of device configuration metrics has been sent;(e) when the complete set of device configuration metrics has been sent,receiving a permanent device identifier from the website; and (f)writing the permanent device identifier to the storage of the computingdevice.
 2. The method of claim 1 wherein the storage of the computingdevice comprises storage reserved for a Flash cookie.
 3. The method ofclaim 1 wherein the storage of the computing device comprises one of DOMstorage and HTML 5 storage.
 4. The method of claim 1 wherein thetemporary device identifier is a product of a hashing algorithmoperating on data from the device configuration metrics.
 5. The methodof claim 1 wherein the subset of device configuration metrics consistsof device configuration metrics that: (i) are retrievable from the webbrowser within a time period specified by the website; and (ii) providethe highest probability among all subsets satisfying condition (i) ofuniquely identifying the computing device.
 6. The method of claim 1further comprising, upon receiving the temporary device identifier,identifying the computing device to website based on the temporarydevice identifier.
 7. The method of claim 1 further comprising, uponreceiving the permanent device identifier, identifying the computingdevice to the website based on the permanent device identifier.
 8. Themethod of claim 1 wherein sending a subset of a complete set of deviceconfiguration metrics further comprising: (a) querying the storage ofthe computing device for the subset indicator identifying all deviceconfiguration metrics previously sent to the website; and (b) sending asubset of device configuration metrics not previously sent to thewebsite.
 9. A method for incrementally fingerprinting a computingdevice, responsive to a web browser on the computing device requesting aresource from a website, comprising: (a) executing device-identificationcode on the computing device programmed to send a complete set of deviceconfiguration metrics to the website; (b) interrupting execution of thedevice-identification code after sending a first subset of the deviceconfiguration metrics; (c) receiving a temporary device identifier andsubset indicator from the website; (d) writing the temporary deviceidentifier and subset indicator to the storage of the computing device;(e) resuming execution of the device-identification code; (f) queryingthe storage of the computing device for a subset indicator identifyingall device configuration metrics previously sent to the website; and (g)sending a second subset of device configuration metrics comprisingdevice configuration metrics not previously sent to the website.
 10. Themethod of claim 9 wherein the interrupting execution of thedevice-identification code occurs as a result of one of (i) the webbrowser exiting the website, (ii) the first subset satisfying apredetermined fingerprinting criteria, and (iii) expiration of anallotted execution time.
 11. The method of claim 9 wherein the resumingexecution of the device-identification code occurs as a result of one of(i) the web browser revisiting the website, (ii) the website fulfillingthe resource request, and (iii) the website reaching a predeterminedstage in its rendering sequence.
 12. The method of claim 9 furthercomprising, upon resuming execution of the device-identification code:(a) querying the storage of the computing device for the subsetindicator, and if the subset indicator indicates that all deviceconfiguration metrics in the set have been sent to the website: (b)omitting further retrieving steps; and (c) promoting the temporarydevice identifier to a permanent device identifier by (i) marking thesubset indicator as permanent, and (ii) writing the permanent deviceidentifier to storage on the computing device.
 13. The method of claim 9wherein the storage of the computing device comprises storage reservedfor a Flash cookie.
 14. The method of claim 9 wherein the storage of thecomputing device comprises one of DOM storage and HTML 5 storage. 15.The method of claim 9 wherein the temporary device identifier is aproduct of a hashing algorithm operating on data from the deviceconfiguration metrics.
 16. The method of claim 9 wherein the firstsubset consists of device configuration metrics from the complete set ofdevice configuration metrics that: (a) are retrievable from the webbrowser within a time period specified by the website; and (b) providehighest probability among all subsets satisfying condition (i) ofuniquely identifying the computing device.